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Nutrient Limitation of Primary Productivity in the Southeast Pacific (BIOSOPE Cruise) Sophie Bonnet, C

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Nutrient Limitation of Primary Productivity in the Southeast Pacific (BIOSOPE Cruise) Sophie Bonnet, C Nutrient limitation of primary productivity in the Southeast Pacific (BIOSOPE cruise) Sophie Bonnet, C. Guieu, F. Bruyant, O. Prášil, France van Wambeke, Patrick Raimbault, T. Moutin, C. Grob, M. Y. Gorbunov, J. P. Zehr, et al. To cite this version: Sophie Bonnet, C. Guieu, F. Bruyant, O. Prášil, France van Wambeke, et al.. Nutrient limitation of primary productivity in the Southeast Pacific (BIOSOPE cruise). Biogeosciences, European Geo- sciences Union, 2008, 5 (1), pp.215-225. hal-00330343 HAL Id: hal-00330343 https://hal.archives-ouvertes.fr/hal-00330343 Submitted on 14 Oct 2008 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Biogeosciences, 5, 215–225, 2008 www.biogeosciences.net/5/215/2008/ Biogeosciences © Author(s) 2008. This work is licensed under a Creative Commons License. Nutrient limitation of primary productivity in the Southeast Pacific (BIOSOPE cruise) S. Bonnet1, C. Guieu1, F. Bruyant2, O. Pra´silˇ 3, F. Van Wambeke4, P. Raimbault4, T. Moutin4, C. Grob1, M. Y. Gorbunov5, J. P. Zehr6, S. M. Masquelier7, L. Garczarek7, and H. Claustre1 1Laboratoire d’Oceanographie´ de Villefranche, UMR 7093, CNRS and Universite´ Pierre et Marie Curie, BP 08 06238 Villefranche sur mer Cedex, France 2Dalhousie University–Department of Oceanography, 1355 Oxford Street Halifax, NS, B3H 4J1, Canada 3Institute of Microbiology ASCR, Opatovicky´ mlyn,´ 37981 Trebon and University of South Bohemia, Zamek,´ 37333 Nove´ Hrady, Czech Republic 4Centre d’Oceanologie´ de Marseille - Campus de Luminy, 13288 Marseille Cedex 09, France 5Rutgers University, Institute of marine and Costal Sciences, 71 Dudley road, New Brunswick, N. J. 08901-8521, USA 6University of California Santa Cruz, Ocean Sciences Department, 1156 high street, Santa Cruz, CA 95064, USA 7Station Biologique de Roscoff, UMR 7144, CNRS and Univ. Pierre et Marie Curie, BP 74, 29682 Roscoff Cedex, France Received: 2 July 2007 – Published in Biogeosciences Discuss.: 9 August 2007 Revised: 19 November 2007 – Accepted: 11 January 2008 – Published: 20 February 2008 Abstract. Iron is an essential nutrient involved in a variety 1 Introduction of biological processes in the ocean, including photosynthe- sis, respiration and dinitrogen fixation. Atmospheric depo- The production of organic matter in the sea is sustained by sition of aerosols is recognized as the main source of iron a continuous supply of essential macro- (C, N, P) and mi- for the surface ocean. In high nutrient, low chlorophyll ar- cronutrients (metals, vitamins). The nutrients requirements eas, it is now clearly established that iron limits phytoplank- vary among different phytoplanktonic species. According to ton productivity but its biogeochemical role in low nutrient, the Liebig’s law, organic matter production is controlled by low chlorophyll environments has been poorly studied. We the element that is available in the lowest concentration rel- investigated this question in the unexplored southeast Pa- ative to the growth requirements. This simple view is now cific, arguably the most oligotrophic area of the global ocean. replaced by the realization that multiple resources simultane- Situated far from any continental aerosol source, the atmo- ously limit phytoplankton growth in some parts of the ocean spheric iron flux to this province is amongst the lowest of (Arrigo, 2005). Global environmental forcings, including the world ocean. Here we report that, despite low dissolved human-induced climate change, could potentially modify the −1 iron concentrations (∼0.1 nmol l ) across the whole gyre nutrient delivery processes to the ocean, leading to funda- (3 stations located in the center and at the western and the mental changes in the diversity and functioning of the ma- eastern edges), primary productivity are only limited by iron rine food web. It is thus fundamental to understand which availability at the border of the gyre, but not in the center. nutrients control primary productivity in the open ocean to The seasonal stability of the gyre has apparently allowed for predict the biogeochemical consequences of global change. the development of populations acclimated to these extreme Representing 60% of the global ocean’s area, the subtropi- oligotrophic conditions. Moreover, despite clear evidence cal open-ocean ecosystems are the largest coherent biomes of nitrogen limitation in the central gyre, we were unable on our planet, and the biogeochemical processes they sup- to measure dinitrogen fixation in our experiments, even af- port are of global importance (Emerson et al., 1997; Karl, ter iron and/or phosphate additions, and cyanobacterial nif H 2002). The development of permanent time series stations gene abundances were extremely low compared to the North in the North tropical Atlantic and Pacific over the past two Pacific Gyre. The South Pacific gyre is therefore unique with decades have led to a revolution in the understanding of the respect to the physiological status of its phytoplankton pop- mechanisms and controls of nutrient dynamics in these re- ulations. mote environments. These oceanic gyres provide ideal eco- logical niche for the development of nitrogen-fixing organ- isms (e.g. Karl et al., 2002). In the North subtropical and Correspondence to: S. Bonnet tropical Atlantic and Pacific oceans, it has been estimated ([email protected]) that dinitrogen fixation is equivalent to 50–180% of the flux Published by Copernicus Publications on behalf of the European Geosciences Union. 216 S. Bonnet et al.: The nutritional status of the South East Pacific a organisms have been suggested to be Fe-limited (Falkowski et al., 1998; Berman-Frank et al., 2001; Moore et al., 2002), 1 but direct experiment were lacking. 2 3 4 6 The BIOSOPE cruise provided the first spatially extensive 7 experiment in the Southeast Pacific. We conducted nutri- 12 13 14 15 ent addition bioassays, designed to investigate which nutri- ent (N, P and/or Fe) controls primary productivity, photosyn- thetic efficiency and dinitrogen fixation along a trophic gra- dient in the Southeast Pacific. A complementary paper (Van b Wambeke et al. this issue) examines the factors that control heterotrophic bacterial growth in the same area. 2 Material and methods Fig. 1. (a) Transect of the BIOSOPE cruise from the Marquesas Is- This research was carried out onboard the R/V Atalante in lands to Chile superimposed on a SeaWiFS surface Chl-a compos- October–November 2004. The experiments were performed ite image (November–December 2004), and location of the short at three stations (Fig. 1a) located in the western edge (sta- (numbers) and long-stations of the cruise (MAR, HNL, GYR, EGY, tion HNL, 9◦ S 136◦ W), in the center (station GYR, 26◦ S UPW, UPX). This study reports results of bioassay experiments per- 114◦ W) and in the southeastern edge of the gyre (station formed at stations HNL, GYR and EGY. (b) Dissolved iron concen- EGY, 34◦ S 92◦ W). trations (0–400 m) (nmol l−1) along the BIOSOPE transect from Marquesas Islands (left) to Chile (right). All experimental setups were performed under strict trace metal clean conditions inside a clean container. Seawater was collected at 30 m depth using a trace metal-clean Teflon pump system and dispensed into 21 to 33 (depending on the − of NO3 into the euphotic zone (Karl et al., 1997; Capone number of treatments) acid-washed 4.5 l transparent polycar- et al., 2005), demonstrating that a large part of new primary bonate bottles. Under a laminar flow hood, nutrients or dust − productivity is fuelled by N2 fixation, rather than NO3 dif- were added either alone or in combination: +Fe, +NPSi and fusing from deeper layer into the euphotic zone. Dinitrogen +FeNPSi at station HNL, +Fe, +N, +P, +FeN, +FeNP and fixation requires the iron-rich nitrogenase complex, there- +dust at stations GYR and EGY. The final concentrations fore N2-fixing organisms have high iron (Fe) requirements −1 + −1 − −1 were 1 µmol l NH4 , 2 µmol l NaNO3 , 0.3 µmol l compared to phytoplankton growing on nitrate or ammonium −1 −1 NaH2 PO4, 2 nmol l FeCl3 and 0.25 mg l of dust. De- (Raven, 1988; Kustka et al., 2003). Dinitrogen fixation in spite the fact that Saharan dust deposition events are unlikely oceanic gyres has been seen to be controlled by Fe avail- to occur in the Southeast Pacific, the dust used in this exper- ability, as well as phosphate, which can (co)limit the process iment was the Saharan soils collected and characterized by (Sanudo-Wilhelmy et al., 2001; Mills et al., 2004). However, Guieu et al. (2002) in order to allow a comparison with ear- all studies dedicated to the nutrient control of primary pro- lier efforts (Mills et al., 2004; Blain et al., 2004, Bonnet et ductivity and dinitrogen fixation have focused so far on the al., 2005). Each fertilization was performed in triplicate. The Northern Hemisphere and there are few data available in the bottles were immediately capped with parafilm, sealed with Southern Hemisphere. PVC tape, and incubated for 48 h in an on-deck incubator The South Pacific gyre, which is the largest oceanic gyre of with circulating surface seawater at appropriated irradiance the global ocean, had been particularly undersampled (Claus- (50% ambient light level). For each station, the incubation tre et al. see introduction of this issue). This unique environ- started in the morning. After two selected time points during ment appears from satellite imagery and ocean colour to have the course of the experiment (T1=24 h; T2=48 h), three repli- the lowest chlorophyll-a (Chl-a) concentrations of the global cates of each treatment were sacrificed in order to measure ocean and thus represents an end-member of oceanic hyper- the following parameters: variable fluorescence, Chl-a con- oligotrophic conditions (Claustre and Maritorena, 2003).
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